The principal values of the chemical shift tensors of all ^13C and ^15N sites in two antibiotics, ampicillin and penicillin-V, were determined by 2-dimensional phase adjusted spinning sideband (2-D PASS) and conventional CP/MAS experiments. The ^13C and ^15N chemical shift anisotropies (CSA), and their confidence limits, were evaluated using a Mathematica program. the CSA values suggest a revised assignment of the 2-methyl ^13C sites in the case of ampicillin. We speculate on a relationship between the chemical shift principal values of many of the ^13C and ^15N sites and the B-lactam ring conformation.